Aligning a distorted image

Abstract

Disclosed herein is a non-transitory computer readable medium that has stored therein a computer program, wherein the computer program comprises code that, when executed by a computer system, instructs the computer system to perform a method of determining the charging induced distortion of a SEM image, the method comprising: determining, at each of a plurality of locations in a SEM image of at least part of a sample, a deflection of an illuminating charged particle beam caused by a charging of the sample at the location; and determining the charging induced distortion of the SEM image in dependence on the determined deflections at each of the plurality of locations in the SEM image.

1 . A non-transitory computer readable medium that has stored therein a computer program, wherein the computer program comprises code that, when executed by a computer system, instructs the computer system to perform a method of determining a charging induced distortion of a SEM image, the method comprising:
 determining, at each of a plurality of locations in a SEM image of at least part of a sample, a deflection of an illuminating charged particle beam caused by a charging of the sample at the location; and   determining the charging induced distortion of the SEM image in dependence on the determined deflections at each of the plurality of locations in the SEM image.   
     
     
         2 . The non-transitory computer readable medium according to  claim 1 , wherein the charging of the sample is caused by the illumination of the sample by one or more charged particle beams. 
     
     
         3 . The non-transitory computer readable medium according to  claim 1 , wherein the SEM image corresponds to a field of view of a charged particle apparatus. 
     
     
         4 . The non-transitory computer readable medium according to  claim 1 , wherein each determined deflection is determined by calculating an electrostatic deflection. 
     
     
         5 . The non-transitory computer readable medium according to  claim 1 , wherein determining each deflection comprises determining both a deflection direction and a deflection magnitude. 
     
     
         6 . The non-transitory computer readable medium according to  claim 1 , wherein determining each deflection comprises modelling each deflection by a charging induced deflection model. 
     
     
         7 . The non-transitory computer readable medium according to  claim 6 , wherein input parameters of the charging induced deflection model comprise one or more of: a dose of an illumination process, dose per pixel in the SEM image, scan speed of the illumination process, scanning pattern, pattern density, pattern parameters, field of view size, pixel size, overscan amount, landing energy of the charged particles, or a separation between the sample and a charged particle source during the illumination process. 
     
     
         8 . The non-transitory computer readable medium according to  claim 6 , wherein variable parameters of the charging induced deflection model comprise one or more of: estimated charging at locations on the sample, estimated charge amplitudes at locations on the sample, translation offsets, charge decay times, interactions between different charge regions or interactions between charges and matter. 
     
     
         9 . The non-transitory computer readable medium according to  claim 6 , the method further comprising:
 determining an imaging error in dependence on features in the SEM image; and   applying the charging induced deflection model to the determined imaging error;   wherein one or more variable parameters of the charging induced deflection model are determined by iteratively comparing the charging model to the imaging error a plurality of times, with the one or more variable parameters adjusted in each iteration.   
     
     
         10 . The non-transitory computer readable medium according to  claim 9 , wherein the imaging error is a local placement error of features in the SEM image. 
     
     
         11 . The non-transitory computer readable medium according to  claim 9 , wherein the imaging error is determined in dependence on a difference between a measured centroid in a SEM image and an expected centroid in the SEM image. 
     
     
         12 . The non-transitory computer readable medium according to any of  claim 9 , the method further comprising processing the imaging error; wherein:
 the charging model is applied to the processed imaging error to generate a reduced imaging error; and   processing the imaging error comprises one or more of:   applying a 6-parameter polynomial fit correction to the imaging error;   applying a mean subtraction to the imaging error;   generating an averaged imaging error in dependence on a 6-parameter polynomial fit correction applied to each of a plurality of SEM images; or   generating an averaged imaging error in dependence on a mean subtraction applied to each of a plurality of SEM images.   
     
     
         13 . The non-transitory computer readable medium according to  claim 9 , the method further comprising applying a 6-parameter polynomial fit correction to the charging model before the charging model is applied to the imaging error, or processed imaging error. 
     
     
         14 . A non-transitory computer readable medium that has stored therein a computer program, wherein the computer program comprises code that, when executed by a computer system, instructs the computer system to perform a method of reducing charging induced distortion of a SEM image, the method comprising:
 determining the charging induced distortion of the SEM image, the determining comprising:
 determining, at each of a plurality of locations in a SEM image of at least part of a sample, a deflection of an illuminating charged particle beam caused by a charging of the sample at the location, and 
 determining the charging induced distortion of the SEM image in dependence on the determined deflections at each of the plurality of locations in the SEM image; and 
   using the determined charging induced distortion to reduce the charging induced imaging error in the SEM image.   
     
     
         15 . A system for determining a charging induced distortion of a SEM image, the system comprising the computer readable medium according to  claim 1 . 
     
     
         16 . A computer-implemented method of determining a charging induced distortion of a SEM image, the method comprising:
 determining, at each of a plurality of locations in a SEM image of at least part of a sample, a deflection of an illuminating charged particle beam caused by a charging of the sample at the location; and   determining the charging induced distortion of the SEM image in dependence on the determined deflections at each of the plurality of locations in the SEM image.   
     
     
         17 . The method according to  claim 16 , wherein the charging of the sample is caused by the illumination of the sample by one or more charged particle beams. 
     
     
         18 . The method according to  claim 16 , wherein the SEM image corresponds to a field of view of a charged particle apparatus. 
     
     
         19 . The method according to  claim 16 , wherein the SEM image comprises a plurality of grids; and each one of the plurality of locations in the SEM image is one of the grids. 
     
     
         20 . The method according to  claim 16 , wherein each determined deflection is determined by calculating an electrostatic deflection.